Epoxy compositions and their curing techniques are well known, and the patents issued on curable epoxy compositions number in the hundreds. It will be appreciated that each and every one of the known epoxy-curing systems exhibits advantages over other systems, and, as importantly, disadvantages over the same systems. There is, of course, a continuing need to develop better epoxy compositions.
There is, in particular, an increasing need in the aerospace and automotive sectors for high performance thermosetting compositions or matrices for fiber reinforced composites. Fiber reinforced composites are very desirable in aerospace applications because they can offer a combination of good stiffness, strength and are light weight. Increasingly, the aerospace manufacturers have demanded higher performance from the thermoset resins used in fiber reinforced composites. These higher performance thermoset resins are expected to possess these following characteristics:
good mechanical properties at temperatures above about 90.degree. C. PA1 good thermal oxidative stability PA1 good toughness properties, including good impact resistance PA1 good fatigue properties PA1 good chemical and solvent resistance PA1 good fire resistance PA1 high resistance to humidity, e.g., the "hot-wet" properties of the composite must remain high. PA1 (a/a+b) is between zero and one; PA1 Glass transition temperature, Tg=175.degree. C. (DSC) PA1 Fracture toughness, K.sub.q =2.5 KSI .sqroot.in (Compact Tension) ##EQU3## Water gain, .DELTA.W/W.sub.o =1.45% (saturation) PA1 SU'/FU' of 2/1 PA1 SU/FU of 5/0 and a ratio of SU'/FU' of 2/1 PA1 SU/FU&gt;SU'/FU'+0.5. PA1 SU/FU&gt;SU'/FU'.
A further, and very important property of such systems is that the composite must have acceptable processing characteristics. For example, the current techniques for manufacturing aerospace components typically involves the use of prepregs and laminates, which are cured by applying heat and pressure in a vacuum bag/autoclave-type apparatus. Therefore, the most desirable thermosetting resin compositions should be processable on the standard equipment currently utilized in the aerospace industry.
A broad spectrum of thermosetting epoxy resin systems is currently being used by the aerospace industry, primarily as composite matrices and adhesives. As a class, epoxy resin systems are very versatile materials offering, as mentioned above, chemical resistance, high adhesive strength, good electric properties and are easy to use or process into composites. However, to improve their high temperature properties, such current epoxy resin systems must be highly crosslinked. This crosslinking, however, results in generally lower toughness.
There are currently various engineering thermoplastics (not thermosetting polymers) that offer excellent high temperature properties along with high toughness. One such example currently being investigated is the use of poly(ether-ether)ketone ("PEEK") as the matrix. However, there are processing problems with the use of PEEK and other similar engineering thermoplastic resins since such materials not only are difficult to process on thermoplastic apparatus (i.e., difficult to extrude), but also do not lend themselves to processing by the thermosetting techniques now currently in use by the aerospace industry.
What is needed is a resin system that only combines the good property advantages of such high performance engineering thermoplastics such as PEEK, but is also processable as a thermosetting resin matrix.